Looking for some help with an exterior light quandary...

Recently purchased a home built in 1901. On the driveway side of the house there is an older existing floodlight with motion sensor. The motion sensor is unfortunately rubbish with no predictability to how it'll go on.

I purchased a new flood to replace it, but when I got up close I realized a problem: the existing flood is mounted into a normal 1-gang rectangular electrical box. The new light is designed to mount into a round box, and after a bit of searching, that seems to be how all the lights on the market are.

Replacing the electrical box with a round one is a bigger project than I'm looking to tackle right now, especially since the house has aluminum siding.

I was thinking maybe they make some kind of adapter plate in between the light and the existing box, but I haven't found one.

Any suggestions on how to handle this?